Cashew nut shell liquid (CNSL), which is a by product of the cashew industry, plays a significant role in the search for cost effective materials which are also renewable. Double distillation of CNSL yields cardanol—a phenol, having a C15 side chain that contributes flexibility, chemical resistance and adhesion in materials prepared from it. Due to this versatile nature, it finds applications in many areas like surface coatings, epoxy, varnishes, paints, printing inks, phenolic resins, rubber compounding, lacquers, laminates, friction materials, and adhesives. In general, CNSL and cardanol are used as phenolic resins by condensation with active hydrogen containing compounds like formaldehyde. In addition, the unsaturation in the C15 side chain makes it amenable to free radical polymerization in presence of commonly available thermal initiators and also to UV initiated polymerization in presence of suitable photoinitiators. However, the double bonds in the long alkyl side chain are not as reactive compared to an acrylic or methacrylic double bond. Functionalization of cardanol with acrylate or methacrylate moiety gives rise to cross linked polymers as reported in the U.S. Pat. No. 6,765,079 by Saminathan, M. et. al. However, cardanyl acrylate and methacrylate based crosslinkers are usually not very stable and often result in the formation of gels. Moreover, photopolymerization leads to shrinkage and the resultant stress causes defects in the bond line and in the resin matrix in the case of fully UV cured (meth)acrylate systems. Compared to free radical cured acrylate resins, polyurethane systems do not show shrinkage related stress. Dual cure coatings involving urethane acrylates combine the benefits of UV curing with that of a two component urethane system and overcome some of their limitations. The cured films are usually more flexible and adhere better to the substrate than 100% UV curable systems, probably because shrinkage is reduced. Acrylic resins with urethane side groups are capable of hydrogen bonding and they exhibit very fast polymerization rates as shown in Macromolecules, 36, (2003) 3861 by Jansen, J. F. G. A et al. Thus acrylic resins with urethane linkage have the added advantage of hydrogen bonding which leads to organization of the monomer units, which in turn increases the rate of polymerization, as illustrated in Eur. Polym. J. 41, (2004), 23 by Asha, S. K. et al. Although reports concerning polyurethanes based on cardanol formaldehyde resin are known, there are not many reports on urethanes synthesized directly from cardanol. Bhunia, H. P. et al. have reported the functionalization of cardanol to obtain a diazo monomer which upon further reaction with diisocyanates gave rise to polyurethane in the J. Polym. Sci: Part A: Polym Chem, 36 (1998) 391. The synthesis of polyurethanes using hydroxy alkylated cardanol formaldehyde as well as cardanol-lignin-formaldehyde resins are reported and the introduction of the urethane functionality has been shown to improve the thermal stability and mechanical properties of the final material as evidenced in the following references. Tan, T. T. M. Polym. International, 41 (1996) 13; Sathiyalekshmi, V et. al Adv. Polym. Tech. 23 (2004) 91. Some patents reported on the development of polymeric materials based on phenolic resin mixed with polyisocyanates and additives are also given in Japanese patent publications (1). Akio, I.; Shigetoshu, A.; Isao, K.; Kkazuuo, T. 62-241914, 1987. (2). Noriimasa, Y.; Noratake, S. 63-97641, 1988. (3). Tsutomu, N.; Koichi, K.; Yoshi, M.; Yuki. 63-213512, 1988. (4). Hisamitsu, I.; Yukio, S. 63-264616, 1988. (5). Laitar; Robert U.S. Pat. No. 4,698,377, 1987.
Thus, cardanol is known to have various potential industrial uses such as resins, surface coatings etc, and development of new applications or improvement of properties of materials based on cardanol by way of new molecular design is strongly desired. Therefore, it is advantageous to have coating compositions containing pre polymers based on cardanol, which combines the added advantage of having a urethane acrylic linkage incorporated in them. In the light of the foregoing discussion, the applicant proposes a UV curable telechelic urethane acrylic pre polymer, based on cardanol or derivative of cardanol.